linux-kernel - RE: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for unlock sequence

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Date:   Wed, 24 Aug 2022 10:05:54 +0000
From:   "Alice Guo (OSS)" <alice.guo@....nxp.com>
To:     Marco Felsch <m.felsch@...gutronix.de>
CC:     Guenter Roeck <linux@...ck-us.net>,
        "wim@...ux-watchdog.org" <wim@...ux-watchdog.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>
Subject: RE: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for
 unlock sequence


> -----Original Message-----
> From: Marco Felsch <m.felsch@...gutronix.de>
> Sent: Wednesday, August 24, 2022 5:06 PM
> To: Alice Guo (OSS) <alice.guo@....nxp.com>
> Cc: Guenter Roeck <linux@...ck-us.net>; wim@...ux-watchdog.org;
> shawnguo@...nel.org; s.hauer@...gutronix.de; festevam@...il.com;
> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> dl-linux-imx <linux-imx@....com>; kernel@...gutronix.de;
> linux-watchdog@...r.kernel.org
> Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory barrier for
> unlock sequence
> 
> Hi Alice,
> 
> On 22-08-24, Alice Guo (OSS) wrote:
> > > -----Original Message-----
> > > From: Marco Felsch <m.felsch@...gutronix.de>
> > > Sent: Wednesday, August 24, 2022 4:04 PM
> > > To: Alice Guo (OSS) <alice.guo@....nxp.com>
> > > Cc: Guenter Roeck <linux@...ck-us.net>; wim@...ux-watchdog.org;
> > > shawnguo@...nel.org; s.hauer@...gutronix.de; festevam@...il.com;
> > > linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org;
> > > dl-linux-imx <linux-imx@....com>; kernel@...gutronix.de;
> > > linux-watchdog@...r.kernel.org
> > > Subject: Re: [PATCH 2/7] watchdog: imx7ulp: Add explict memory
> > > barrier for unlock sequence
> > >
> > > Hi Alice,
> > >
> > > On 22-08-24, Alice Guo (OSS) wrote:
> > >
> > > ...
> > >
> > > > > > > Hi Guenter and Marco,
> > > > > > >
> > > > > > > 1. did you see any issues?
> > > > > > > This WDOG Timer first appeared in i.MX7ULP, no one report
> > > > > > > issues probably because few people use i.MX7ULP. This issue
> > > > > > > was found when we did a stress test on it. When we
> > > > > > > reconfigure the WDOG Timer, there is a certain probability
> > > > > > > that it reset. The reason for the error is that when
> > > > > > > WDOG_CS[CMD32EN] is 0, the unlock sequence is two 16-bit
> > > > > > > writes (0xC520, 0xD928) to the CNT register within 16 bus
> > > > > > > clocks, and improper unlock sequence causes the
> > > WDOG to reset.
> > > > > > > Adding mb() is to guarantee that two 16-bit writes are
> > > > > > > finished within 16
> > > > > bus clocks.
> > > > > >
> > > > > > After this explanation the whole imx7ulp_wdt_init() seems a
> > > > > > bit buggy because writel_relaxed() as well as writel() are
> > > > > > 32bit access
> > > functions.
> > > > > > So the very first thing to do is to enable the 32-bit mode.
> > > > > >
> > > > > Agreed. This is much better than having extra code to deal with
> > > > > both 16-bit and 32-bit access.
> > > > >
> > > > > > Also this is a explanation worth to be added to the commit
> > > > > > message
> > > > > > ;)
> > > > > >
> > > > >
> > > > > Definitely. Also, the use of mb(), if it should indeed be
> > > > > needed, would have to be explained in a code comment.
> > > > >
> > > > > Thanks,
> > > > > Guenter
> > > >
> > > > Hi Marco and Guenter,
> > > >
> > > > Thank you for your comments. I plan to enable support for 32-bit
> > > > unlock command write words in bootloader. In this way, there is no
> > > > need to distinguish whether the unlock command is a 32-bit command
> > > > or a 16-bit command in driver.
> > >
> > > Please don't move this into the bootloader, enabling it within the
> > > init seq. is just fine. If you move it into the bootloader then you
> > > can't ensure that the bit is set since there are plenty of bootloaders out
> there.
> > >
> > > As I said, just drop the "16bit" unlock sequence from the init
> > > function because the unlock is handled just fine in all the watchdog_ops.
> > >
> > > Regards,
> > >   Marco
> >
> > Hi Marco,
> >
> > Sorry, I did not tell you that all watchdog control bits, timeout
> > value, and window value cannot be set until the watchdog is unlocked.
> 
> You don't have to according the RM:
> 8<----------------------------------------------------------------------
> 59.5.2 Disable Watchdog after Reset
> 
> All of watchdog registers are unlocked by reset. Therefore, unlock sequence is
> unnecessary, but it needs to write all of watchdog registers to make the new
> configuration take effect. The code snippet below shows an example of
> disabling watchdog after reset.
> 8<----------------------------------------------------------------------
> 
> also the RM tells us:
> 8<----------------------------------------------------------------------
> 59.4.3.1 Configuring the Watchdog Once
> 
> The new configuration takes effect only after all registers except CNT are
> written after reset. Otherwise, the WDOG uses the reset values by default. If
> window mode is not used (CS[WIN] is 0), writing to WIN is not required to
> make the new configuration take effect.
> 8<----------------------------------------------------------------------
> 
> > Support for 32-bit unlock command write words in enabled in
> > imx7ulp_wdt_init now.
> 
> So.. after reading the IMX7ULP RM, which was not my intention, I found out
> that most of the WDOG_CS regiter bits are write-once bits. This means if you
> didn't set it within the bootloader you still in case "59.4.3.1".
> 
> So the imx7ulp_wdt_init() function just needs to check if the
> WDOG_CS_UPDATE bit was set. If it is not the case, then you need to write the
> WDOG_CS register as currently done. If the bit is set, than you need know that
> the bootloader did the job for you and you can exit
> imx7ulp_wdt_init() early. In both cases the unlock is not required.
> 
> Can you please check/test if this is working for you?
> 
> Regards,
>   Marco
> 

Hi Marco,

Rom code has already configured the WDOG once, so we cannot use " Configuring the Watchdog Once".

Best Regards,
Alice Guo